Spherical Mechanism Synthesis in Virtual Reality - Florida Institute ...

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positions have been placed, the spherical constraint surface is defined and displayed. The remaining two positions are confined to the sphere defined by the first two position choices. After four positions are placed, the user selects the desired order in which the mechanism should move through them. At this point, the fixed and moving axis cones or the type map may be generated. Isis gives the position information to the Sphinx computation routines, and the Sphinx algorithms return the appropriate information. The ability to work with geometric models is a feature that sets Isis apart from previous approaches to design. Motion synthesis within the context of its application allows users to see right away whether or not a mechanism is feasible for its intended use. Unwanted collisions between objects can instantly be seen, and the design can be altered immediately. Figure 3 depicts four instances of a tray model placed around a chair that will act as a base for a mechanism. It can clearly be seen that the intended task of the mechanism in this example will be to move the tray from its working position to its stowed location. This would not be evident if a user were to work exclusively with coordinate frames, as can be seen in Figure 4, which shows the same four positions in Sphinx. The design emphasis in Isis is placed on defining the task of moving the tray table, as opposed to placing coordinate frames on a constraint sphere. Figure 4 - Tray Positions in Sphinx Figure 5 shows another example of a spherical motion synthesis task that benefits from using geometric models. A designer can create a mechanism that ensures that the soda can will travel along a path that is free of obstruction. Figure 5 - Soda Can Task Cones Sphinx routines calculate the fixed and moving axis cones, and Isis displays these as red and blue 3-D curves. Users of Isis are able to see links appear as selections are made on the cones. When two links are chosen, a coupler curve appears, and a message box opens to tell the user what type of linkage has been synthesized. The user may then move the axes to see how the linkage and coupler curve change. Figure 6 shows a user creating a link by picking on the red fixed axis cone. Figure 6 - Picking Cones 4 Copyright © 1998 by ASME
Type Map As stated previously, a type map is a 2-D plot that displays the solutions generated by Burmester's theory color-coded by mechanism type. Isis takes advantage of the VR display and brings the type map into three dimensions. Linkages that pass the test of input drivability and reach the orientations in the desired order are shown on a plane above the whole solution set, and those solutions that do not pass the tests are darkened on the type map. The result of this is a display that shows good and bad mechanisms together, with the good mechanisms easily distinguishable from the undesirable ones. A user selects a point in a colored region based on the type of linkage that he or she is trying to synthesize, and the linkage corresponding to that point appears along with information about the linkage type and folding condition. In the example shown in Figure 7, one can see the type map, which displays the better mechanisms as areas of bright blue and yellow. Additionally, a complete mechanism resulting from the type map choice is shown with its coupler curve depicted in yellow. To the right of the mechanism is a message box containing confirmation of the mechanism type that has been chosen and information about the folding condition of the linkage. Figure 7 - Type Map Interaction Design Verification Once a mechanism has been synthesized in Isis, it may be animated to verify that it completes the required task. The transparent positions remain present, and an opaque instance of the moving geometry moves along with the coupler link of the mechanism. Designers can observe how smoothly a mechanism moves, and they can see whether or not objects will collide during motion of the mechanism. The virtual reality interface allows a designer to move around the design and investigate the motion of the mechanism from many different angles. It is very intuitive to investigate the operation of the mechanism because the designer’s viewpoint changes according to natural human motions. File Manipulation After a user has synthesized a mechanism, an output file can be generated. The file written by Isis is an augmented Sphinx file, which is readable by Sphinx. Isis can load a Sphinx file as well as its own file type, which contains information about the placement of geometric models within the environment in addition to the data that Sphinx uses. Summary of Program Usage Figure 8 shows a flowchart summarizing the procedure taken to synthesize a spherical mechanism in Isis. Parallel portions of the flowchart indicate optional paths. For example, a user may bring in geometry or simply use coordinate frames to define a synthesis task. Alternatively, a mechanism previously saved from Sphinx or Isis may be loaded into the program. To generate or redesign a mechanism, either cones or a type map may be used. Users may alter positions and regenerate the cones or type map until a good solution is obtained. In Figure 8, the gray boxes indicate parts of the program that utilize Sphinx functions. Some of the Sphinx functions have remained untouched, but others have been modified to fill Isis data structures and create geometry for display by WorldToolKit. Figure 8 - Diagram of Program Usage 5 Copyright © 1998 by ASME
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